1. Field of the Invention
The invention relates to telephone receiving equipment which is adapted to recognize out of incoming signals a number of different frequency combinations, of a number of nominal voice-frequency signalling components. To this end such a receiving equipment contains a number of mutually different Discrete Fourier Transform (DFT) digital signal-processing devices each having a frequency response characteristic the top of whose main lobe corresponds to one of such nominal voice-frequency signalling components situated within one of two frequency band subgroups into which such voice-frequency signalling components are divided; and a processor which processes the results provided by the digital signal processing devices to detect the two nominal voice-frequency signalling components that are received with the greatest strength, and evaluates the other of such results so as to provide a voice-signal guard function.
2. Description of the Related Art
Such a receiving equipment is known from published Netherlands patent application 8200051, which corresponds to U.S. Pat. No. 4,479,229. This describes an equipment for receiving multifrequency code (MFC) signals and which is provided with DFT signal processing devices in which provisions are made to secure immunity against single spurious signal frequencies. These signal-guard provisions are not suitable for voice-signal protection. Such devices are further equipped so as to reduce to a minimum the extra computing time required.
U.S. Pat. No. 4,355,405 describes a receiver for push-button signalling (TDK). Such receiver comprises eight different digital signal-processing devices in the form of digital filters for the eight voice-frequency signalling components that are customary in such a signalling system. Such filters are implemented in this case in the form of "Finite Impulse Response" filters whose results are used on the one hand for detecting the relevant frequency combinations of signalling components, and on the other hand for fulfilling a voice-signal guard function. In this prior art it is proposed to add a side lobe to each filter transfer characteristic. The frequencies corresponding to the tops of these side lobes are chosen in such a way that one side-lobe top frequency is added concurrently to the four pairs of nominal pushbutton signalling frequencies, i.e. the pairs 697, 770; 852, 941; 1209, 1336; and 1477, 1633. Such side lobe top frequencies are 1040 for the first pair, 640 for the second pair, 580 for the third pair and 1100 for the fourth pair. A filter system implemented in this way has the drawback that there are gaps in the voice-frequency spectrum which it analyses. This means that certain voice or spurious signal components, or a combination of nominal signalling components, can produce imitations or cause the rejection of an incoming validating combination of nominal signalling components. Furthermore the selectivity of the filter characteristics thus divided over the frequency range is unsatisfactory. Nor does the literature referred to deal with provisions that would provide immunity in such pushbutton signalling against single spurious signal frequencies.